I don't know the reason behind this decision, but if it's anything along the lines of a design decision that left 6v6 and Rumble out deliberately, Bungie needs to change that asap. If it's a "we'd like to add those things, but we're way too busy", then they should just come out and say it. These are game types that NEED to be in Destiny 2. We could probably live with having to wait a bit until they're ready, but the least we deserve is confirmation that they'll be going in at some point.